On Some Ore Deposits of the Kuroko (Black Ore) Type in the Western Part of the Kosaka Mining District

Abstract

Several ore-deposits such as Nagaki and Onemori are found in the western part of the Kosaka Mine, Akita Prefecture, Japan. The geology of the environment of this district is charactrized by the "green tuff" formation derived from the submarine volcanism in the Miocene period. The Kagoya formation conformably overlying the Nagaki formation prevails extensively in this district. Both formations are composed of acidic pyroclastic rocks and mudstone, though the constituent rocks of the Nagaki formation suffered stronger alteration than those of the Kagoya formation. The Nagaki formation is intruded by several rhyolite masses of dike-or stock-form. The Nagaki and the Onemori ore deposits are composed of copper-lead-zinc veins and gypsum deposits which belong to the "Kuroko" type.
The majority of veins occur in fractures of rhyolite masses intruding the Nagaki formation. Un the other hand, gypsum deposits occur with the vein type deposits in the uppermost part of the Nagaki formation. No ore-deposits are found in the Kagoya formation.
This fact may be reasonably explained as follows : Before the sedimentation of the Kagoya formation, two types of ore-deposits were, formed in connection with the shallow-seated intrusion of rhyolite. One type is epigenetic metal vein and the other syngenetic gypsum bed.